In a known double cutter boring tool, the head part 1 carries two cutting bit holders 2 and 3, which in turn carry interchangeable bits. The head end of the boring tool, which is trapezoidal in the lateral view of FIG. 3, has a linear tooth system on the sloping lateral faces 6, 6a along which the bit holders 2, 3 provided with an identical tooth system can be moved in such a way that the cutting edges of the bits can be adjusted radial in a direction towards and away from the turning centre. The bore diameter is set in this way. By means of a working adjustment range measured from the turning centre, there is a reciprocal supporting of the two bit holders in such a way that even very high cutting forces do not lead to inelastic permanent deformation of the bit holders, or even to the splitting thereof, i.e. to their fracture. If the adjustment range is so large that it is outside the reciprocal support of the bit holders, i.e. in ranges where the material cross-section of the bit holder absorbs the cutting forces and the much lower feed forces via the tooth systems on the head part 1, such undesired occurrences can be encountered.
In order to be able to obtain a desired minimum boring diameter in the case of completely slid together cutting bit holders, at the maximum the support faces 8, 8a assume the displacement length of the minimum radius, i.e. the distance from the turning centre to the circumference. Beyond this radial adjustment path, there is no longer any reciprocal supporting of the bit holders. In this adjustment range, there must no longer be a maximum loading of the tool, which directly influences the metal removal rate and consequently the profitability.